Most types of packages like the plastic encapsulated (PEL) package exert mechanical stress on a semiconductor die supplied by the package. Reasons for such mechanical stress are versatile and may be based on, e.g., a mismatch of thermal coefficients of expansion (CTE) of the package constituents (mold compound, semiconductor die, leadframe), a moisture uptake or a chemical cure shrink of organic parts of the package (mold compound, adhesives). The mechanical stress may be of significant size, e.g., amount up to 300 megapascal (MPa) and beyond. The semiconductor die itself may be subject to compression (i.e. negative stress), but the use of die coatings, glob tops or ceramic packages may lead to tension (i.e. positive stress).
Stress is risk factor affecting the reliability of a circuitry and it may alter parameters of electronic components of the semiconductor die. This can reduce, e.g., an accuracy of Hall sensor circuits, bandgap circuits, temperature sensors, on-chip relaxation oscillators or current references.
A linear relation between a stress vector T and a normal vector n follows from the fundamental laws of conservation of linear momentum and static equilibrium of forces. The components of the Cauchy stress tensor at every point in a material satisfy the equilibrium equations (Cauchy's equations of motion for zero acceleration). Moreover, the principle of conservation of angular momentum implies that the stress tensor is symmetric. Therefore, the stress state of the medium at any point and instant can be specified by only six independent parameters, rather than nine, i.e.,
      [                                        σ            xx                                                σ            xy                                                σ            xz                                                            σ            xy                                                σ            yy                                                σ            yz                                                            σ            xz                                                σ            yz                                                σ            zz                                ]    ,
wherein the elements σxx, σyy, σzz are referred to as orthogonal normal stresses (relative to the chosen coordinate system), and the elements σxy, σxz, σyz are referred to as orthogonal shear stresses.
Due to the laminar structure of the package, the main portion of the die surface has dominant stress components amounting to σxx and σyy (in case the surface of the die is arranged in a x-y-plane). For such dies, in particular for the circuits that are provided by the die, it is beneficial to monitor or measure the mechanical stress.